Add IsValid property to ValidationStateChangedEventArg

[vendasankarsf3945]

Include valid/invalid state in ValidationStateChangedEventArgs

Description

ValidationStateChangedEventArgs currently does not expose whether the affected field is in a valid or invalid state. Consumers handling EditContext.OnValidationStateChanged must re-query validation messages to determine the field’s status.
This PR updates ValidationStateChangedEventArgs to include the current valid/invalid state, allowing event subscribers to react directly to validation outcomes without additional lookups.

Before

• EditContext.OnValidationStateChanged only indicated that validation state had changed.
• ValidationStateChangedEventArgs did not contain any information about whether the field was valid or invalid.
• Consumers were required to:
  • Call EditContext.GetValidationMessages(...), or
  • Perform additional validation checks to infer validity.

After

• ValidationStateChangedEventArgs includes an IsValid property indicating the field’s validation state.
• EditContext.OnValidationStateChanged subscribers can directly determine validation results from the event arguments.
• This simplifies validation-related logic, reduces redundant validation queries, and improves developer ergonomics.

Fixes #13413

[vendasankarsf3945]

@dotnet-policy-service agree company="Syncfusion, Inc."

[dotnet-policy-service]

Thanks for your PR, @vendasankarsf3945. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.